Molds such as molds for injection molding, molds for die casting, and molds for hot pressing (also called as hot stamping or die quenching), which are used for resins, rubbers, etc., have conventionally been produced generally by melting a steel to form an ingot thereof, thereafter subjecting the ingot to forging and rolling to form a block or a flat rectangular material, machining this material into a shape of a mold, and then giving thereto heat treatments such as quenching and tempering.
A generally employed technique concerning these molds is to form a cooling circuit (water cooling line) within the wall of the mold and pass cooling water therethrough to thereby cool the mold.
In such molds, to heighten the efficiency of cooling with cooling water results in a reduction in cycle time, i.e., fast-cycle production (molding) of products, and this leads to an improvement in production efficiency.
A direct method for heightening the efficiency of cooling is to dispose the cooling circuit nearer to the molding surface (design surface) of the mold.
However, this method has the following drawback. Due to the reduced distance between the cooling circuit and the molding surface and due to the generation of stronger thermal stress, the mold is prone to develop a severe crack (through crack propagation from the water cooling circuit to the molding surface), which is causative of a decrease in mold life.
Consequently, in cases when the cooling circuit is disposed nearer to the molding surface, there are limitations as a matter of course.
Another possible method may be one in which a cooling circuit that runs complicatedly meanderingly in all directions is formed within the wall of the mold to heighten the cooling ability by regulating the overall shape of the cooling circuit, layout thereof, etc. However, with any method in which a mold is produced through machining, it is technically impossible to form a cooling circuit having such a complicated shape.
Under such circumstances, attention is recently being focused on a technique for manufacturing a mold by additive manufacturing (three-dimensional additive manufacturing).
Additive manufacturing is a technique of processing for converting a three-dimensional model data into an actual object by accumulation of a material. In additive manufacturing, a shape expressed by three-dimensional computer aided design (CAD) data is first sliced along a plurality of planes perpendicular to a predetermined axis, and the sectional shapes of the resultant slices are calculated. The shapes of these slices are actually formed, and the formed slices are stacked and bonded together, thereby converting the computer-expressed shape into an actual object.
In additive manufacturing, there are cases where a powder is used as the material and cases where plates are used as the material.
In the method in which a powder is used as a material, the powder is evenly spread into a layer (each layer has a thickness of, for example, several tens of micrometers), and certain regions of the powder layer are irradiated with thermal energy, for example, irradiated with a laser beam of an electron beam, to melt/solidify or to sinter the powder layer. Layers are thus superposed one by one to thereby fabricate a whole shape.
Meanwhile, in additive manufacturing in which plates are used as a material, individual parts (plates) resulting from the slicing of three-dimensional model data in a CAD are actually produced by machining, etc., and these parts are stacked and bonded together by, for example, diffusion bonding, thereby manufacturing a whole three-dimensional shape.
Examples of mold production by these additive manufacturing techniques are disclosed, for example, in Patent Documents 1 and 2.
Specifically, Patent Document 1 discloses an invention relating to “a metal powder for selective laser sintering, a method for manufacturing a three-dimensional shaped object by using the same, and the three-dimensional shaped object obtained therefrom.” Disclosed therein is a feature of forming a solidified layer by irradiating a predetermined portion of a layer of a powdery material including a precipitation-hardening metal composition with a light beam, thereby allowing sintering of the powder of the predetermined portion or melting and subsequent solidification thereof, and forming another solidified layer by newly forming a powder layer on the resulting solidified layer, and then irradiating another predetermined portion of the new powder layer with the light beam, these steps being repeatedly performed, to thereby produce a three-dimensional shaped object.
Patent document 2 discloses an invention relating to “a cavity insert for mold, a method for manufacturing an insert for mold, and a resin molding mold.” Disclosed therein is a feature that a cavity insert having a spiral cooling passage inside is produced based on slice data of the cooling passage by processing a groove which forms the cooling passage in each of a plurality of metal plates, laminating the groove-processed metal plates in a prescribed order, diffusion-bonding the laminated metal plates, and shape-processing a metal block obtained by the diffusion bonding.
The techniques of additive manufacturing described above are ones that fabricate a whole shape by stacking a material, and are capable of easily forming a complicated cooling circuit which runs meanderingly in all directions and which cannot be formed by machining at all. As a result, the efficiency of cooling can be effectively rendered higher than that of molds produced by conventional machining, without the need of disposing the cooling circuit unnecessarily close to the molding surface of the mold.
Hitherto, maraging steels and precipitation hardening-type stainless steels have been used as a material for molds required to have high-temperature strength.
In patent document 1 also, powders of a maraging steel or a precipitation hardening-type stainless steel are hence used as materials for molds.
Although such steels including maraging steels and precipitation hardening-type stainless steels have high-temperature strength sufficient for molds, there is a problem in that these steels have low heat conduction performance (low coefficient of thermal conductivity) since the matrix phase thereof contains elements which are prone to form a solid solution, such as Si, Cr, Ni, and Co, in a large amount.
Molds produced by additive manufacturing have advantages in that a cooling circuit having a freely designed complicated shape can be disposed therein and that consequently even a mold to be produced by using a maraging steel or precipitation hardening-type stainless steel as a material therefor can be made to have a heightened cooling efficiency due to the shape effect of the cooling circuit formed to have a complicated shape by additive manufacturing. However, since the material itself has a low coefficient of thermal conductivity, it is difficult to heighten the efficiency of cooling to a sufficient level.
It is a matter of course that in cases where a mold is produced therefrom not by additive manufacturing but by a conventional general production method, the efficiency of cooling (heat exchange) becomes more insufficient.
Meanwhile, there are carbon steels, steels for mechanical structural use, and the like as steels having high heat conduction performance (having a high coefficient of thermal conductivity). These steels show high heat conduction performance since the contents of elements which are prone to form a solid solution, such as Si, Cr, Ni, and Co, in the matrix phase are low and since these steels are low-alloy steels.
However, these steels have low high-temperature strength and have a problem in that the molds produced therefrom have a short life.
Namely, there has been no steel provided so far for molds which is capable of giving a mold having sufficient performance in terms of both high-temperature strength and heat conduction performance, regardless of whether or not the mold is manufactured by additive manufacturing.
As a prior-art technique relevant to the present invention, Patent Document 3 discloses an invention relating to “a die steel having excellent thermal fatigue properties”. Disclosed therein is a feature that the addition amounts of Si and Cr, which are alloying elements, are reduced and other alloying components are balanced, thereby attaining an increase in the coefficient of thermal conductivity and an increase in softening resistance.
As another prior-art technique, Patent Document 4 discloses an invention concerning “a steel for die”. Disclosed therein is a feature that the addition amounts of Si, Mn, and Cr are properly balanced to thereby effectively regulate the coefficient of thermal conductivity of the steel to a value not less than a desired value and to sufficiently ensure machinability and impact value.
As another further prior-art technique, Patent Document 5 discloses an invention relating to “a die steel superior in spheroidizing annealing property and hardenability.” Disclosed therein is a feature that by regulating the elements to be added to a steel, both hardenability and spheroidizing annealing property, which are required for large molds of 500 kg or more, are imparted to the steel.
The components of each of the steels described in Patent Documents 3 to 5 may partially overlap the components of the steel for a mold of the present invention with respect to the range of chemical components specified in their claims. However, there is no Example disclosed therein, which satisfies any of the claims of the present invention, and the steels of Patent Documents 3 to 5 substantially differ from that of the present invention.
In addition, the steels described in Patent Documents 3 to 5 are not intended to be used in additive manufacturing, and this use is not mentioned therein at all.                Patent Document 1: WO 2011/149101        Patent Document 2: JP-A-2010-194720        Patent Document 3: Japanese Patent No. 4,992,344        Patent Document 4: JP-A-2011-94168        Patent Document 5: JP-A-2008-121032        